1. Field of the Invention
The present invention relates to an in-focus detection apparatus installed in a video camera, etc. which uses a video signal output from an image sensing device for in-focus operation.
2. Description of the Related Art
Conventionally, cameras used for electrically sensing the image of an object such as a video camera, etc. use an automatic focusing apparatus which detects whether or not an in-focus state is reached using a video signal and uses the detection signal. Most of such devices take advantage of the fact that the high frequency component in the video signal reaches its maximum in the in-focus state. Other apparatuses have been proposed, which conduct in-focus operation using an edge width as an evaluation value, taking into account that the width of the edge portion of an object is shortest in the in-focus state.
FIG. 19 is a representation showing the relation between edge widths and in-focus state. FIG. 19(A) represents the in-focus state, while (B) represents the out-of-focus state. In FIG. 19, frame 30 represents an AF area. For example, in the in-focus state shown in (A), the distribution of the amount of light received by a CCD at a certain edge portion in the AF area is as shown at the bottom. Portions defined by boxes represent the pixels of CCD. In the in-focus state, change from dark (amount of received light being small) to light (amount of received light being large) or change from light to dark takes place with a very small number of pixels. The change of the amount of light received by the CCD in this case is set forth in the center by a graphic representation. In the figure, .DELTA. represents rising time at the edge portions of an object, and .DELTA. is defined as the edge width of the object. On the other hand, if the object is in the out-of-focus state, as shown in (B), the edge width .DELTA.' is larger than that in the case of in-focus state (A).
As described above, according to the method utilizing the high frequency component, the direction in which the high frequency component increases is detected by constantly slightly moving a focus lens, and the position at which the high frequency component reaches its maximum is searched while moving the lens in this direction. And driving of the focus lens is stopped at that position. This method is called mountain climbing method.
According to this method, the focusing lens should be slightly moved all the time. Also, the absolute amount of the peak to be detected is not constant, and, therefore, it is not possible to calculate how much the position of the focus lens at present is shifted from the in-focus state (defocus amount). Furthermore, information obtained by accumulating the amount of light received by the CCD in the area is used, instead of video signals obtained independently for CCD pixels, thus making it difficult to increase in-focus accuracy.
Also according to the method, if an object with edges arranged adjacent to each other like a lattice is sensed in a largely out of focus state, the contrast sometimes indicates the maximum value despite the largely out of focus state, due to the contrast inverted portion of photographing lens MTF characteristic (pseudo peak). According to the mountain climbing method, driving of the focus lens is stopped at this pseudo peak position, and accurate in-focus detection is not possible.
Meanwhile, the method using edge widths can advantageously utilize CCD information. The focusing accuracy is far higher than the above-stated high frequency component method. Furthermore, defocus amounts can be estimated from the detected edge widths.
Although this method is greatly effective when used to sense an ideal object which has edges of one kind, an actual object usually has a various kinds of edge components in the AF area and it is therefore not easy to determine on which edge portion the in-focus determination should be concentrated.
Also in a largely out of focus state, adjacent edges interfere with each other, thus making it difficult to calculate an accurate defocus amount.